Modulation of ligand binding in engineered human hemoglobin distal pocket

Functional and structural studies on hemoglobin and myoglobin from different animals and engineered variants have enlightened the great importance of the physico-chemical properties of the side-chains at topological position B10 and E7. These residues proved to be crucial to the discrimination and stabilisation of gaseous ligands. Ln view of the data obtained on the high oxygen affinity hemoglobin from Ascaris worms and a new mutant of sperm whale myoglobin, we selected the two mutations Leu B10 --> Tyr and His E7 --> Gin as potentially relevant to control ligand binding parameters in the alpha and beta-chains of human hemoglobin. Here, we present an investigation of three new mutants: Hb alpha YQ (alpha(2)(YQ)beta(2)(A)), Hb beta YQ (alpha(2)(A)beta(2)(YQ)) and Hb alpha beta YQ (alpha(2)(YQ)beta(2)(YQ)). They are characterised by a very low reactivity for NO, O-2 and CO, and a reduced cooperativity. Their functional properties are not inconsistent with the behaviour expected for a two-state allosteric model. Proteins with these substitutions may be considered as candidates for the synthesis of a possible "blood substitute", which should yield an O-2 adduct stable to autoxidation and slowly reacting with NO. The mutant Hb alpha beta YQ is particularly interesting because the rate of reaction of NO with the oxy and deoxy derivatives is reduced. A structural interpretation of our data is presented based on the 3D structure of deoxy Hb alpha beta YQ determined by crystallography at 1:8 Angstrom resolution. (C) 1999 Academic Press.